1. Technical Field
The present invention relates to an optical head which causes a current-drive-type light-emitting element, which is driven by a current to emit light, to emit light, thereby forming a latent image on an image carrier, and an image forming apparatus which forms an image using the optical head,
2. Related Art
The above optical head is configured such that a plurality of unit circuits each including a current-drive-type light-emitting element used as a light source, and a driving transistor (an active element, such as a thin-film transistor) which generates a driving current are arrayed in one direction. A variation in characteristics may exist in the driving transistor. This tendency becomes significant when the driving transistor is a thin-film transistor. If such a variation exists, there is a possibility that an unevenness may occur in the gray-scale of a latent image. Thus, a technique of compensating (correcting) the variation in the characteristics of the driving transistor to reduce the unevenness in the gray-scale is suggested (refer to JP- A-2002-144634).
The current-drive-type light-emitting element has a temperature characteristic of emitting light with high luminance, if the temperature thereof is high. For example, even if the gray-scale data that specifies turn-on (high gray-scale level) is supplied to the same unit circuit two times, the light-emitting element will emit with different luminances if the temperatures of the light-emitting element when the light-emitting element of the unit circuit emits light according to the gray-scale data differ from each other. This leads to an unevenness in the gray-scale of a latent image. Accordingly, in an optical head used for an image forming apparatus, the temperature of the light-emitting element is kept constant during a period when an image is formed.
The temperature of the light-emitting element fluctuates according to a heat quantity which is obtained by subtracting the quantity of heat radiated from the light-emitting element by heat conduction from the applied heat quantity of the light-emitting element, i.e., the quantity of heat applied to the light-emitting element. Among the heat quantities applied to the light-emitting element, a large heat quantity is applied when the light-emitting element is used as a heat source, and a large heat quantity is applied when the driving transistor which drives the light-emitting element is used as a heat source. These heat quantities are generated while the light-emitting element is turned on, but are not generated while the light-emitting element is turned off. Accordingly, in order to keep the temperature of the light-emitting element constant, certain measures need to be taken. However, such measures have yet to be implemented.
Further, in the optical head, unit circuits are arranged in proximity to each other so as to be adjacent to each other. Thus, a heat quantity applied to a light-emitting element of a certain unit circuit (hereinafter referred to as “first unit circuit”) also includes a heat quantity where a light-emitting element or a driving transistor of a unit circuit (referred to as “second unit circuit”) adjacent to the first unit circuit is used as a heat source. As the resolution of an image to be formed is higher, that is, unit circuits adjacent to each other are closer to each other, this heat quantity becomes larger. Further, this heat quantity is generated while the light-emitting element of the second unit circuit is turned on, but is not generated while the light-emitting element is turned off. Therefore, in order to keep the temperature of the light-emitting element constant, certain measures need to be taken. However, such measures have not yet, to be implemented.